Message-ID: <CAEf4BzYd=2=RiHXNkhQ+67QdGB+K2foHQsZVeCXGC6kkE4MjMQ@mail.gmail.com>
Date:   Thu, 9 Jul 2020 15:02:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH bpf-next v3 01/16] bpf, netns: Handle multiple link attachments

On Thu, Jul 9, 2020 at 5:49 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Thu, Jul 09, 2020 at 05:44 AM CEST, Andrii Nakryiko wrote:
> > On Thu, Jul 2, 2020 at 2:24 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
> >>
> >> Extend the BPF netns link callbacks to rebuild (grow/shrink) or update the
> >> prog_array at given position when link gets attached/updated/released.
> >>
> >> This let's us lift the limit of having just one link attached for the new
> >> attach type introduced by subsequent patch.
> >>
> >> No functional changes intended.
> >>
> >> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> >> ---
> >>
> >> Notes:
> >>     v3:
> >>     - New in v3 to support multi-prog attachments. (Alexei)
> >>
> >>  include/linux/bpf.h        |  4 ++
> >>  kernel/bpf/core.c          | 22 ++++++++++
> >>  kernel/bpf/net_namespace.c | 88 +++++++++++++++++++++++++++++++++++---
> >>  3 files changed, 107 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> >> index 3d2ade703a35..26bc70533db0 100644
> >> --- a/include/linux/bpf.h
> >> +++ b/include/linux/bpf.h
> >> @@ -928,6 +928,10 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array *progs,
> >>
> >>  void bpf_prog_array_delete_safe(struct bpf_prog_array *progs,
> >>                                 struct bpf_prog *old_prog);
> >> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
> >> +                                  unsigned int index);
> >> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
> >> +                             struct bpf_prog *prog);
> >>  int bpf_prog_array_copy_info(struct bpf_prog_array *array,
> >>                              u32 *prog_ids, u32 request_cnt,
> >>                              u32 *prog_cnt);
> >> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> >> index 9df4cc9a2907..d4b3b9ee6bf1 100644
> >> --- a/kernel/bpf/core.c
> >> +++ b/kernel/bpf/core.c
> >> @@ -1958,6 +1958,28 @@ void bpf_prog_array_delete_safe(struct bpf_prog_array *array,
> >>                 }
> >>  }
> >>
> >> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
> >> +                                  unsigned int index)
> >> +{
> >> +       bpf_prog_array_update_at(array, index, &dummy_bpf_prog.prog);
> >> +}
> >> +
> >> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
> >> +                             struct bpf_prog *prog)
> >
> > it's a good idea to mention it in a comment for both delete_safe_at
> > and update_at that slots with dummy entries are ignored.
>
> I agree. These two need doc comments. update_at doesn't event hint that
> this is not a regular update operation. Will add in v4.
>
> >
> > Also, given that index can be out of bounds, should these functions
> > actually return error if the slot is not found?
>
> That won't hurt. I mean, from bpf-netns PoV getting such an error would
> indicate that there is a bug in the code that manages prog_array. But
> perhaps other future users of this new prog_array API can benefit.
>
> >
> >> +{
> >> +       struct bpf_prog_array_item *item;
> >> +
> >> +       for (item = array->items; item->prog; item++) {
> >> +               if (item->prog == &dummy_bpf_prog.prog)
> >> +                       continue;
> >> +               if (!index) {
> >> +                       WRITE_ONCE(item->prog, prog);
> >> +                       break;
> >> +               }
> >> +               index--;
> >> +       }
> >> +}
> >> +
> >>  int bpf_prog_array_copy(struct bpf_prog_array *old_array,
> >>                         struct bpf_prog *exclude_prog,
> >>                         struct bpf_prog *include_prog,
> >> diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
> >> index 247543380fa6..6011122c35b6 100644
> >> --- a/kernel/bpf/net_namespace.c
> >> +++ b/kernel/bpf/net_namespace.c
> >> @@ -36,11 +36,51 @@ static void netns_bpf_run_array_detach(struct net *net,
> >>         bpf_prog_array_free(run_array);
> >>  }
> >>
> >> +static unsigned int link_index(struct net *net,
> >> +                              enum netns_bpf_attach_type type,
> >> +                              struct bpf_netns_link *link)
> >> +{
> >> +       struct bpf_netns_link *pos;
> >> +       unsigned int i = 0;
> >> +
> >> +       list_for_each_entry(pos, &net->bpf.links[type], node) {
> >> +               if (pos == link)
> >> +                       return i;
> >> +               i++;
> >> +       }
> >> +       return UINT_MAX;
> >
> > Why not return a negative error, if the slot is not found? Feels a bit
> > unusual as far as error reporting goes.
>
> Returning uint played well with the consumer of link_index() return
> value, that is bpf_prog_array_update_at(). update at takes an index into
> the array, which must not be negative.

Yeah, it did, but it's also quite implicit. I think just doing
BUG_ON() for update_at or delete_at would be good enough there.

>
> But I don't have strong feelings toward it. Will switch to -ENOENT in
> v4.
>
> >
> >> +}
> >> +
> >
> > [...]

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